Wood chippers are commonly used for reducing large pieces of wood into smaller pieces. Some wood chippers are drum-type wood chippers having a rotating working drum equipped with blades or knives. In these types of wood chippers, the working drum may rotate at thousands of revolutions per minute to chop up and/or shred the wood. In this type of wood chipper, the working drum is generally located within an enclosure. Since there are times when access to the working drum within the enclosure is necessary for maintenance or repair, or in the event of failure of the chipper, it is common for wood chippers and similar devices to include an access door that permits access to the interior of the enclosure.
It is known to provide electronic components for drum-type wood chippers that are designed to sense the rotation of the working drum and to block the access door from opening while the working drum is rotating. While these electronic components may be suitable in many instances, various drawbacks may exist. For example, electronic components may be sensitive to heat and moisture extremes that may be characteristic of the environment in which such wood chippers are generally used. In addition, wood chippers may encounter or cause vigorous vibrations which may also cause electronic controls to fail or to operate unreliably.